videoio: add Retrieve function

videoio.cpp

@@ -47,6 +47,10 @@ void VideoCapture_Grab(VideoCapture v, int skip) {
     }
 }
 
+int VideoCapture_Retrieve(VideoCapture v, Mat buf) {
+    return v->retrieve(*buf);
+}
+
 // VideoWriter
 VideoWriter VideoWriter_New() {
     return new cv::VideoWriter();

videoio.go

@@ -379,6 +379,14 @@ func (v *VideoCapture) Grab(skip int) {
 	C.VideoCapture_Grab(v.p, C.int(skip))
 }
 
+// Retrieve decodes and returns the grabbed video frame. Should be used after Grab
+//
+// For further details, please see:
+// http://docs.opencv.org/master/d8/dfe/classcv_1_1VideoCapture.html#a9ac7f4b1cdfe624663478568486e6712
+func (v *VideoCapture) Retrieve(m *Mat) bool {
+	return C.VideoCapture_Retrieve(v.p, m.p) != 0
+}
+
 // CodecString returns a string representation of FourCC bytes, i.e. the name of a codec
 func (v *VideoCapture) CodecString() string {
 	res := ""

videoio.h

@@ -28,6 +28,7 @@ double VideoCapture_Get(VideoCapture v, int prop);
 int VideoCapture_IsOpened(VideoCapture v);
 int VideoCapture_Read(VideoCapture v, Mat buf);
 void VideoCapture_Grab(VideoCapture v, int skip);
+int VideoCapture_Retrieve(VideoCapture v, Mat buf);
 
 // VideoWriter
 VideoWriter VideoWriter_New();

videoio_test.go

@@ -199,3 +199,37 @@ func TestVideoWriterFile(t *testing.T) {
 		t.Error("Invalid Write() in VideoWriter")
 	}
 }
+
+func TestVideoCaptureFile_GrabRetrieve(t *testing.T) {
+	vc, err := VideoCaptureFile("images/small.mp4")
+	defer vc.Close()
+
+	if err != nil {
+		t.Errorf("%s", err)
+	}
+
+	if !vc.IsOpened() {
+		t.Error("Unable to open VideoCaptureFile")
+	}
+
+	if fw := vc.Get(VideoCaptureFrameWidth); int(fw) != 560 {
+		t.Errorf("Expected frame width property of 560.0 got %f", fw)
+	}
+	if fh := vc.Get(VideoCaptureFrameHeight); int(fh) != 320 {
+		t.Errorf("Expected frame height property of 320.0 got %f", fh)
+	}
+
+	vc.Set(VideoCaptureBrightness, 100.0)
+
+	vc.Grab(10)
+
+	img := NewMat()
+	defer img.Close()
+
+	if ok := vc.Retrieve(&img); !ok {
+		t.Error("Unable to read VideoCaptureFile")
+	}
+	if img.Empty() {
+		t.Error("Unable to read VideoCaptureFile")
+	}
+}